A very good reason for multiple brokers is so that you can run different screens in different accounts.

I use Fidelity for everything except my HSA brokerage account (when I set it up they didn't handle them, maybe they do now). Fidelity lets me open accounts however I wish. I have a different accounts for different things. My "scholarship fund" is separate from my retirement funds separate from my other designated funds. So I do run different screens in different accounts in effect.

The only advantage I'd see to different brokers, insurance issues. Like bank accounts with FDIC, brokers are supposed to be covered by SIPC. It to has its limits. However, when I hit those limits, I talked with Fidelity and they said they buy extra insurance or something to that effect. So it depends if your comfortable with the "extra insurance" of if you want to depend on the government.

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